1. Field of the Invention
The present invention relates to a so-called tandem type image forming apparatus. More particularly, the present invention relates to formation and detection of test patterns used for adjusting image forming conditions.
2. Description of the Related Art
In image forming apparatuses, such as printers, digital copiers, and facsimile, that form an image by an electrophotographic method, a method of writing images on a photosensitive body using a laser beam scanning has become mainstream in recent years. The method includes periodically scanning (main scanning) a laser, in which lighting is controlled by video (line image) signals, with a beam scanning optical system. The optical system includes a polygon mirror and the like. The laser is projected onto the photosensitive body on a surface to be scanned, and the photosensitive body is generally moved in a direction perpendicular to the main scan (sub-scan) direction. By using such a scanning exposure, the two dimensional image is drawn on the photosensitive body.
An electrostatic latent image formed on the photosensitive body by the scanning exposure is then developed by a toner, transferred directly on a recording (transfer) paper or transferred via an intermediate transfer medium, and fixed. By executing each step, an image forming process is completed.
When a color image is formed by carrying out a step of scanning exposure using a light beam as the above, the scanning exposure of the photosensitive body is performed by each color component of the color image. Accordingly, the color image is formed through a process of synthesizing each color component. In the related art, a method that a single photosensitive body is commonly used to each color component, and performs the color synthesizing at the step of scanning exposure (write) or a step of transferring is known. Another known method is a so-called tandem type that provides photosensitive bodies for each color component, writes respective color component images to each photosensitive body, and performs the color synthesizing at the step of transferring thereafter.
In the tandem type image forming apparatuses, the color synthesizing is performed by performing scanning exposure on the respective photosensitive bodies of each color component. As a result, positional deviations and density deviations tend to occur between the respective color component images being synthesized. A step of image forming needs to be managed so that these deviations do not occur. Therefore, by detecting the states of the image forming system of each color component, and adjusting the image forming conditions and the image forming system corresponding to a change in the states, a proper image output should be obtained.
A conventionally known method that detects states of the image forming system adopted in the tandem type, forms test patterns by actually operating the image forming system of each color component under a predetermined condition. The formed test patterns of each color component are read, thereby detecting deviation amounts from the read result. After detecting this state, control amounts are corrected based on the detected deviation amounts, thereby controlling the image forming system to operate properly. “Correction patterns” hereinafter described indicate the test patterns.
In the method of forming the correction patterns and detecting the states of the image forming system in the related art, when the image forming system is operated properly, one example is a method of forming the correction patterns of each color on a conveyor belt of a transfer paper or an intermediate transfer belt. The correction patterns are formed in the main and the sub-scanning directions in a predetermined position relationship. Then, errors are calculated from the deviation amounts from the predetermined position appearing on the correction patterns. For example, Japanese Patent Application Laid-Open No. H11-249380 and Japanese Patent Application Laid-Open No. 2004-287403 are exemplified as the ones adopting a method of forming correction toner marks. The correction toner marks are formed at two detection positions on the conveyor belt in the main scanning direction and may detect the deviation amounts in the respective main scanning and the sub-scanning directions. In these patent documents, a method of respectively detecting an inclination (skew), a resist in the sub-scanning direction, a resist in the main scanning direction, and a magnification error in the main scanning direction that cause the positional deviations between each color component image is shown. The control amounts such as an image write start timing is adjusted according to the detection result, thereby operating the image forming system properly.
In Japanese Patent No. 3644923 and Japanese Patent Application Laid-Open No. 2004-101567, an example of a method that is basically the same as the Japanese Patent Application Laid-Open No. H11-249380 and the Japanese Patent Application Laid-Open No. 2004-287403, but intended to further improve the detection accuracy is shown. In the method, correction toner marks are formed at three detection positions on the conveyor belt in the main scanning direction. The correction toner marks may detect the deviation amounts in the respective main scanning direction and the sub-scanning direction.
Further, in the Japanese Patent No. 3644923, not only detecting the toner marks for positional-deviation correction, but also a configuration of forming density correction toner marks (patches) of each color, and commonly using a detecting unit for the positional-deviation correction to detect the density correction toner marks is adopted. Here, in a scanning exposing unit, a writing timing of an image, a drive of the photosensitive body, an exposure, and the like, are adjusted according to detected amounts of the positional deviation. In a toner developing unit, a developing bias and a charging bias are adjusted according to the detected amounts of the density deviation.
However, in the conventional methods that detect the states of the image forming system by detecting each patterns of the positional-deviation correction and the density correction, as shown in the Japanese Patent No. 3644923, formation of each of the correction patterns and detection process of the patterns are performed separately.
In other words, while correction operation is performed, the conveyor belt, the intermediate transfer belt, or the like forming the correction patterns are only used for the correction. Therefore, the processing time for the positional-deviation correction and the density correction need to be added up for the required processing time. The longer processing time of the conventional method hampers the processing speed.